1. Field of the Invention
This invention relates to an apparatus for transferring ink from an ink ribbon to a recording medium, by applying heat to the medium, thereby recording data on the recording medium, and more particularly, to a so-called "thermal recording printer."
2. Description of the Related Art
An apparatus, generally known as a "thermal recording printer", transfers ink from an ink ribbon to a recording medium, by heating the ink ribbon and thereby melting the ink. The printer can print data on sheets of ordinary paper, without making much noise, and can operate very reliably. For these advantages, the thermal recording printer is used as hard copy printers for use in various OA (Office Automation) apparatuses such as personal computers, word possessors, and color printers. The thermal recording printer is disadvantageous in two respects. First, the ink ribbon is liable to be cut during use. Secondly, the printer cannot print data in sufficient quality, on sheets of coarsely textured paper such as PPC paper or bond paper.
FIG. 1 is a schematic view showing an electrothermal printer of the known type. In this printer, use is made of ink ribbon 1 comprised of electrically resistive base film 2, electrically conductive layer 3 made of aluminum, and solid ink layer 4 coated on conductive layer 3. Ink layer 4 will melt, soften, or sublime when heated. Ink ribbon 1 is fed in the direction of arrow A by means of a ribbon-feeding mechanism (not shown).
As is shown in FIG. 1, the electrothermal printer comprises data-recording electrodes 5, signal-generating circuit 6, and return electrode 7. Electrodes 5 are pin-shaped and arranged parallel to one another. They can be moved in the direction of arrow C, and are electrically coupled with signal-generating circuit 6. Return electrode 7, which is moved along with electrode 5, is connected to the ground and located downstream of the ribbon-feeding direction (arrow A). Return electrode 7 is coupled to follow roller 8 by the ribbon-feeding mechanism. Follow roller 8 contacts ink ribbon 1; it is rotated as the mechanism feeds ink ribbon 1 in the direction of arrow A.
To print data on recording paper 9 located below ink ribbon 1, signal-generating circuit 6 supplies data signals to data-recording electrodes 5. Electrodes 5 supply ink ribbon 1 with the currents corresponding to the data signals. These currents (hereinafter referred to as "data currents") flows through resistive base film 2 into conductive layer 3, and flow from layer 3 to return electrode 7 through resistive base film 2, as is shown by arrow B. As the data currents flows from electrodes 5 through base film 2, Joule heat is generated in the limited portions of ink ribbon 1 which are located below electrodes 5. These portions of ribbon 1 are heated to 200.degree. C. or more, whereby those portions of ink layer 4 which are on these portions of ribbon 1 are softened or melted. As a result, the ink is transferred from ribbon 1 onto recording paper 9.
As has been described above, the data currents also flow to return electrode 7 through resistive base film 2, and change into Joule heat. This heat is not sufficient to melt or soften solid ink layer 4, since that surface of return electrode 7 which contacts the ribbon 1 is much larger than that surface of each data-recording electrode 5 which contacts ribbon 1. Thus, return electrode 7 does not operate to transfer ink onto recording paper 9.
Data-recording electrodes 5 are moved, along with return electrode 7, in the direction of arrow C. While electrodes 5 are thus moved, they supply data currents to ink ribbon 1, in response to the data signals output from signal-generating circuit 6. Therefore, the ink is continuously transferred from ribbon 1 onto recording paper 9, whereby data, such as images and characters, are reproduced on recording paper 9.
As has been described, it is within ink ribbon 1 that heat is generated within ink ribbon 1 during the use of the thermal recording printer. Thus, the heat is fast transmitted to solid ink layer 4, and the printer can record data on paper at a speed higher than ordinary thermal printers having a thermal head which applies heat to an ink ribbon. Since heat is generated within ink ribbon 1, it is applied in its entirety to solid ink layer 4, thus heating layer 4 to a high temperature. Hence, solid ink layer 4 can be made of material having a high melting point or a high sublimation point.
Resistive ink ribbon 1 is made of three layers, and is more difficult to manufacture and, hence, more expensive than the ink ribbon for use in the ordinary thermal printers, which is comprised of two layers, i.e., an electrically resistive base film and a solid ink layer. Another drawback inherent in the resistive thermal printer is that each portion of ink ribbon 1 required for printing one line of characters cannot be shorter than the line of characters, and the running cost of the printer is, thus, relatively high.
A method is disclosed in U.S. Pat. No. 4,558,963 in which an ink ribbon is fed at low speed, in order to use the ink ribbon more efficiently in such a resistance thermal printer as is shown in FIG. 1, and thus to lower the running cost of the printer. Since the tape-feeding speed is low, the ink ribbon will likely be cut. Also, the low speed of feeding the ribbon results in the following problem.
As been explained, in the electrothermal printer shown in FIG. 1, the data currents applied to ink ribbon 1 change into Joule heat in those portions of solid ink layer 4 which are located below electrode 5. Since ribbon 1 is fed slowly, a great amount of heat is generated in these portions of ink layer 4. Those portions of conductive layer 3 and base film 2 which receive this heat are heated to 200.degree. C. or more. As a result, the heated portions of layer 3 may be oxidized or cracked, and the heated portions of base film 2 may shrink. If this happens, all conductive layer 3 rendered almost nonconductive, except for both lateral edges which are not located under electrodes 5. The data currents flow concentratedly through the thin lateral edges of conductive layer 3 into that portion of base film 2 which contacts return electrode 7. When electrode 7 contacts any shrinked portion of resistive base film 2, which is narrower than unshrinked portions, a great amount of Joule heat is generated in the shrinked portion. This heat is transferred to the unshrinked portions of film 2, inevitably softening these portions and also the remaining portions of solid ink layer 4.
Consequently, ink ribbon 1 is cut at such a softened portion of base film 2, overcome by the tension which is applied on that portion of ribbon 1 which extends between data-recording electrodes 5, on the one hand, and return electrode 7, on the other. Moreover, ribbon 1 may be adhered to follow roller 8 by the remaining ink layer 4, now softened and thus viscous, and it may eventually taken up around roller 8. In the worst case, it may be cut at a shrinked portion of base film 2, which is positioned between roller 8 and electrodes 5.
The slower the ribbon is fed, thereby to use the ribbon efficiently, the greater the possibility that the ribbon is cut. Hence, it is practically impossible to apply the method disclosed in U.S. Pat. No. 4,558,963, wherein an ink ribbon is fed at low speed, to the resistance thermal printer having the structure shown in FIG. 1.